This invention relates generally to seals between metal lead-in conductors and fused quartz or fused silica for electrical devices having a sealed envelope with electric current being supplied to electric energy translation means located within the sealed envelope through said lead-in conductors. More particularly, the present invention employs an improved sealing glass composition to provide a novel molten seal between a fused quartz member which is pinch-sealed to a refractory metal inlead that has been inserted into an opening provided in the fused quartz member. Specifically, said improved sealing glass composition melts at elevated temperatures above about 350.degree. C. when the electrical device is operated and provides a molten seal around the inleads which serves to protect against oxidation or contamination of these parts. At ordinary temperatures, however, this sealing glass solidifies to a crazed or frit condition in the opening due to its higher thermal expansion than the thermal expansion of the refractory inlead metal.
Pinch seals are known and commonly employed with various type electric lamps being operated at elevated temperatures up to 500.degree. C. and higher, hence require that a transparent envelope material be employed which is capable of withstanding the operating temperatures. It is also commonly required that said lamp envelopes further be sealed directly to the electrical inlead components, such as now practiced in commercial incandescent type quartz heating lamps as well as discharge lamps to include regenerative cycle halogen and other type discharge lamps. In achieving this latter objective with refractory metal inleads, it is generally required that very thin flat foils be used to preserve a hermetic seal during extended periods of lamp operation. In U.S. Pat. No. 3,211,826, which is assigned to the assignee of the present invention, there is disclosed a representative quartz to metal seal of the type being employed in a commercial quartz infrared lamp. There is also disclosed in another U.S. Pat. No. 3,868,528, assigned to the present assignee, a representative metal halide discharge lamp employing the same type pich-seal construction.
It has now also become desirable to remove lead as well as arsenic from glass materials employed in consumer products to avoid the ecological problems associated with these substances. Accordingly, it is an object of the present invention to provide a lead-free sealing glass composition which enables a novel molten seal to be formed between a refractory metal lead-in conductor and fused quartz when the electric lamp is being operated. It is a further object of the present invention to provide a substitute sealing glass devoid of both lead and arsenic which does not require that a substantial modification be made in the otherwise conventional pinch seal construction of an electric lamp. In achieving these objectives with molybdenum and tungsten lead-in conductors, it is especially critical that oxidation of said metals be avoided by protective action of the sealing glass employed since the seal temperatures often exceed 500.degree. C. and higher during operation of the current lamps.